Added sanity checks to :meth:~.Mobject.add_to_back for Mobjects (#1220)

* Add Mobject add_to_back sanity checks

- Raises ValueError when Mobject tries to add itself
- Raises TypeError when a non-Mobject is added
- Filters out incoming duplicate submobjects if at least one instance of
that submobject exists in the list

* Filter incoming Mobjects into a set before adding

First, incoming list of Mobjects will be filtered into a set with no
duplicates. Then, any mobject in submobjects that is a duplicate of
an incoming mobject will be removed. The filtered set of mobjects will
be added to back and the remaining submobjects are appended.

* Add isort to pre-commit config

* Isort test_vectorized_mobject.py

* Was already added

* Update .pre-commit-config.yaml

Co-authored-by: kolibril13 <[email protected]>
Co-authored-by: Benjamin Hackl <[email protected]>

Author: 3 people

manim/mobject/mobject.py

@@ -353,9 +353,36 @@ def add_to_back(self, *mobjects: "Mobject") -> "Mobject":
             the head of :attr:`submobjects`. The head of this list is rendered
             first, which places the corresponding mobjects behind the
             subsequent list members.
+
+        Raises
+        ------
+        :class:`ValueError`
+            When a mobject tries to add itself.
+        :class:`TypeError`
+            When trying to add an object that is not an instance of :class:`Mobject`.
+
+        Notes
+        -----
+        A mobject cannot contain itself, and it cannot contain a submobject
+        more than once.  If the parent mobject is displayed, the newly-added
+        submobjects will also be displayed (i.e. they are automatically added
+        to the parent Scene).
+
+        See Also
+        --------
+        :meth:`remove`
+        :meth:`add`
+
         """
+        for mobject in mobjects:
+            if self in mobjects:
+                raise ValueError("Mobject cannot contain self")
+            if not isinstance(mobject, Mobject):
+                raise TypeError("All submobjects must be of type Mobject")
+
+        filtered = list_update(mobjects, self.submobjects)
         self.remove(*mobjects)
-        self.submobjects = list(mobjects) + self.submobjects
+        self.submobjects = list(filtered) + self.submobjects
         return self
 
     def remove(self, *mobjects: "Mobject") -> "Mobject":

tests/test_vectorized_mobject.py

@@ -1,6 +1,6 @@
 import pytest
 
-from manim import Mobject, VDict, VGroup, VMobject
+from manim import Line, Mobject, VDict, VGroup, VMobject
 
 
 def test_vgroup_init():
@@ -86,6 +86,24 @@ def test_vgroup_remove_dunder():
     assert len(obj.submobjects) == 0
 
 
+def test_vmob_add_to_back():
+    """Test the Mobject add_to_back method."""
+    a = VMobject()
+    b = Line()
+    c = "text"
+    with pytest.raises(ValueError):
+        # Mobject cannot contain self
+        a.add_to_back(a)
+    with pytest.raises(TypeError):
+        # All submobjects must be of type Mobject
+        a.add_to_back(c)
+
+    # No submobject gets added twice
+    a.add_to_back(b)
+    a.add_to_back(b, b)
+    assert len(a.submobjects) == 1
+
+
 def test_vdict_init():
     """Test the VDict instantiation."""
     # Test empty VDict